United Nations: As many as 170 countries are expected to sign the Paris Agreement on climate change Friday as the landmark deal takes a key step toward entering into force years ahead of schedule.

US Secretary of State John Kerry joins dozens of world leaders for a signing ceremony that is expected to set a record for international diplomacy: Never have so many countries signed an agreement on the first available day. States that don't sign on Friday have a year to do so.

Many now expect the climate agreement to enter into force long before the original deadline of 2020. Some say it could happen this year.

Narendra Modi and Barack Obama at the COP21 summit in Paris. AFP

After signing, countries must formally approve the Paris Agreement through their domestic procedures. The United Nations says at least 13 countries are expected to do that Friday by depositing their instruments of ratification.

The agreement will enter into force once 55 countries representing at least 55 percent of global emissions have formally joined it. The United States and China, which together account for nearly 40 percent of global emissions, have said they intend to join this year.

"We definitely want to be in the first wave of ratifying countries," Maros Sefcovic, the energy chief for another top emitter, the 28-nation European Union, told reporters Thursday.

Countries that had not yet indicated they would sign the agreement Friday include some of the world's largest oil producers, including Saudi Arabia, Iraq, Nigeria and Kazakhstan, the World Resources Institute said Thursday.

The Paris Agreement, the world's response to hotter temperatures, rising seas and other impacts of climate change, was reached in December as a major breakthrough in UN climate negotiations, which for years were slowed by disputes between rich and poor countries over who should do what.

Under the agreement, countries set their own targets for reducing emissions of carbon dioxide and other greenhouse gases. The targets are not legally binding, but countries must update them every five years.

Already, states face pressure to do more. Scientific analyses show the initial set of targets that countries pledged before Paris don't match the agreement's long-term goal to keep global warming below 2°Celsius (3.6°Fahrenheit), compared with pre-industrial times. Global average temperatures have already climbed by almost 1 degree C. Last year was the hottest on record.

The latest analysis by the Climate Interactive research group shows the Paris pledges put the world on track for 3.5°C of warming. A separate analysis by Climate Action Tracker, a European group, projected warming of 2.7°C.

Either way, scientists say the consequences could be catastrophic in some places, wiping out crops, flooding coastal areas and melting Arctic sea ice.

The United States is a key concern for the Paris Agreement as other countries worry what the next president might do. Analysts say that if the agreement enters into force before President Barack Obama leaves office in January, it would be more complicated for his successor to withdraw from the deal, because it would take four years to do so under the agreement's rules.

"Walking away from the agreement would instantly turn the US from a leader to a defector" with serious diplomatic consequences, Elliot Diringer of the US-based Centre for Climate and Energy Solutions think tank told reporters Thursday.

The Obama administration is expected to treat the deal as an executive agreement, which needs only the president's approval.

As the Paris Agreement moves forward, there is some good news. Global energy emissions, the biggest source of man-made greenhouse gases, were flat last year even though the global economy grew, according to the International Energy Agency.

Still, fossil fuels are used much more widely than renewable sources like wind and solar power.